1. Field of the Invention
The present invention relates to a magnetic disk apparatus to be used in an information processing system, and more particularly to a magnetic head positioner which can prevent an arm holder providing a plurality of magnetic heads from causing thermal deformation due to variation of ambient temperature.
2. Description of the Related Art
The detail of a conventional magnetic head positioner of a magnetic disk apparatus has been disclosed in U.S. Pat. No. 5,161,077, for example. The conventional magnetic head positioner of the magnetic disk apparatus will be discussed hereinafter with reference to FIGS. 3 and 4.
FIG. 4 is a graph showing experimental data of thermal off track of the conventional magnetic head positioner at a predetermined ambient temperature. The abscissas axis of the graph shown in FIG. 4 represents an average value of the result of measurement of the thermal off track of the magnetic head at the predetermined ambient temperature T. On the other hand, the ordinate axis of the graph shown in FIG. 4 represents head numbers given to a plurality of magnetic heads shown in FIG. 3 in ascending order from the lowermost one.
Referring to FIG. 3, the conventional magnetic head positioner includes a shaft 101, a plurality of bearings 102, an arm holder 103, arms 104, magnetic heads 105, a voice coil 106 and a sleeve 107.
The shaft 101 is fixedly arranged within the magnetic disk apparatus. The shaft 101 also functions as a center axis for rotation of the magnetic heads and is made of a material including a stainless steel.
The plurality of bearings 102 are arranged in axial direction with maintaining a predetermined distance to the shaft 101.
The arm holder 103 is rotatably supported about the shaft 101 by the bearings 102 in order to rotate thereabout. The plurality of arms 104 protrude perpendicularly from the outer surface of the arm holder 103. Each arm 104 provides the magnetic head 105 at tip end thereof for writing and reading data with respect to a magnetic disk medium 108.
The voice coil 106 is mounted on the outer peripheral surface of the arm holder 103 at the opposite side to the plurality of arms 104. The voice coil 106 also functions to generate a driving force to rotate the arm holder 103 about the shaft 101.
The cylindrical sleeve 107 is interposed between the outer peripheral surfaces of the plurality of bearings 102 and the inner peripheral surface of the arm holder 103.
However, the foregoing conventional magnetic head positioner has some the following disadvantages.
In general, the shaft, the plurality of bearings, the arm holder and the sleeve to be used in the magnetic head positioner are formed of mutually different materials having difference physical characteristics, such as Young's modulus, linear expansion coefficient and so forth.
For instance, when the arm holder and the sleeve are formed of the same material, the shaft and the sleeve may expand in the axial direction depending upon their linear expansion coefficient if the ambient temperature is varied significantly. Therefore, due to difference of linear expansion of the shaft and the sleeve, it is possible to cause releasing of pre-load on the bearings or operation of excess load on the bearings.
On the other hand, when the bearings and the sleeve are formed of the same material, the bearings and the arm holder may cause thermal deformation in magnitudes depending upon their linear expansion coefficients according to rising of the ambient temperature. Since the linear expansion coefficients are different in the sleeve and the arm holder, a thermal stress is generated at the contacting portion between the sleeve and the arm holder to cause thermal off track of the magnetic heads mounted on the arm holder.
Furthermore, with reference to FIG. 4, an absolute value of the thermal off track magnitude of the magnetic head 105 at the predetermined ambient temperature T is approximately 2.3 .mu.m. Also, comparing the thermal off track magnitude of respective magnetic head 105, a difference of the thermal off track magnitude of the head No. 1 and head No. 10 is large. This phenomenon means that the uppermost portion of the arm holder 103 is inclined toward inside of the magnetic disk medium 108 in a greater magnitude than the lowermost portion thereof. On the other hand, the thermal off track magnitudes of the head Nos. 6, 7 and 8 are considerably larger than that of other heads, respectively. This phenomenon means that the central portion of the arm holder 103 extends toward inside of the magnetic disk medium 108 in a greater magnitude than the uppermost and lowermost portions thereof. Namely, the arm holder 103 is deformed in arch shape at the center portion thereof.